Data Communications and
Networking by
Forouzan fourth edition
Chapter 12
Multiple Access
McGraw-Hill
12.1
©The McGraw-Hill Companies, Inc., 2000
Multiple Access
In Chapter 11 we discussed data link control, a mechanism
which provides a link with reliable communication. In the
protocols we described, we assumed that there is an
available dedicated link (or channel) between the sender
and the receiver. This assumption may or may not be true.
If, indeed, we have a dedicated link, as when we connect
to the Internet using PPP as the data link control protocol,
then the assumption is true and we do not need anything
else.
PPP: point-to-point protocol(byte oriented ) which provides
connections over multiple links and has a very simple
mechanism for error control.
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12.2
We can consider the data link layer as two sublayers.

The upper sublayer is responsible for data link control
and error control is called the logical link control (LLC)
layer .
 The lower sublayer is responsible for resolving access to
the shared media is called the media access control
(MAC) layer . If the channel is dedicated, we do not
need the lower sublayer.
 Figure 12.1 shows these two sublayers in the data link
layer. When nodes or stations are connected and use a
common link, called a multipoint or broadcast link, we
need a multiple-access protocol to coordinate access to
the link.
12.3
Figure 12.1 Data link layer divided into two functionality-oriented sublayers
Upper sublayer – responsible for data link control
•Called LLC – for flow and error control
Lower sublayer - responsible for resolving access the shared media
•Called MAC – for multiple acceess resolution
12.4

The problem of controlling the access to the medium
is similar to the rules of speaking in an assembly. The
procedures guarantee that the right to speak is up held
and ensure that two people do not speak at the same
time, do not interrupt each other.

The situation is similar for multipoint networks.
Many formal protocols have been devised to handle
access to a shared link. We categorize them into three
groups. Protocols belonging to each group are shown
in Figure 12.2.
12.5
Figure 12.2 Taxonomy of multiple-access protocols discussed in this chapter
12.6
12-1 RANDOM ACCESS
In random access or contention methods, no station
is superior to another station and none is assigned
the control over another. No station permits, or does
not permit, another station to send. At each instance,
a station that has data to send uses a procedure
defined by the protocol to make a decision on
whether or not to send. The decision depends on the
state of the medium (idle or busy).
Topics discussed in this section:
ALOHA
Carrier Sense Multiple Access
Carrier Sense Multiple Access with Collision Detection
Carrier Sense Multiple Access with Collision Avoidance
12.7
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12.8
Two features give this method its name. First, there is no
scheduled time for a station to transmit. Transmission is random
among the stations. That is why these methods are called random
access. Second, no rules specify which station should send next.
Stations compete with one another to access the medium. That is
why these methods are also called contention methods.
In a random access method, each station has the right to the
medium without being controlled by any other station. However,
if more than one station tries to send, there is an access conflictcollision-and the frames will be either destroyed or modified.
The random access methods we study a very interesting protocol
known as ALOHA, which used a very simple procedure called
multiple access(MA), that forces the station to sense the medium
before transmitting.
ALOHA
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12.9
It was designed for a radio (wireless) LAN, but it can be used on
any shared medium. The medium is shared between the stations.
When a station sends data, another station may attempt to do so
at the same time. The data from the two stations collide .
The original ALOHA protocol is a simple, but elegant protocol.
The idea is that each station sends a frame whenever it has a
frame to send. Figure 12.3 shows an example of frame collisions
in pure ALOHA. Some of these frames collide because
multiple frames are in contention for the shared channel.
Figure 12.3 Frames in a pure ALOHA network
Pure ALOHA:
1. Each station sends a frame whenever is has a frame to send
2. One channel to share, possibility of collision between frames from different stations
12.10
Figure 12.4 Procedure for pure ALOHA protocol
12.11
Figure 12.5 Vulnerable time for pure ALOHA protocol
12.12
Figure 12.6 Frames in a slotted ALOHA network
Slotted ALOHA:
1. We divide the time into slots and force the station to send only at the beginning of
the time slot
12.13
Figure 12.7 Vulnerable time for slotted ALOHA protocol
12.14
Carrier Sense Multiple Access (CSMA)
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12.15
To minimize the chance of collision and, therefore, increase
the performance, the CSMA method was developed. The
chance of collision can be reduced if a station senses the
medium before trying to use it. CSMA requires that each station
first listen to the medium (or check the state of the medium)
before sending. it is based on the principle "sense before
transmit“.
CSMA can reduce the possibility of collision, but it cannot
eliminate it. The possibility of collision still exists because of
propagation delay.
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What should a station do if the channel is busy or idle? Three methods
have been devised are:
I-Persistent The I-persistent method is simple and straightforward. In
this method, after the station finds the line idle, it sends its frame
immediately (with probability I).
This method has the highest chance of collision because two or more
stations may find the line idle and send their frames immediately,
Ethernet uses this method.
Nonpersistent In the nonpersistent method, a station that has a frame
to send senses the line. If the line is idle, it sends immediately. If the
line is not idle, it waits a random amount of time and then senses the
line again. The nonpersistent approach reduces the chance of collision
because it is unlikely that two or more stations will wait the same
amount of time and retry to send simultaneously. However, this
method reduces the efficiency of the network because the medium
remains idle when there may be stations with frames to send.
12.16

p-Persistent The p-persistent method is used if the
channel has time slots with a slot duration equal to or
greater than the maximum propagation time.

The p-persistent approach combines the advantages
of the other two strategies. It reduces the chance of
collision and improves efficiency.
12.17
Figure 12.10 Behavior of three persistence methods
1-Persistent-after station
finds the line idle, send
its frame
Nonpersistent-senses the
line; idle: sends
immediately; not idle:
waits random amount of
time and senses again
p-Persistent-the channel
has time slots with
duration equal to or
greater than max
propagation time
12.18
Figure 12.11 Flow diagram for three persistence methods
12.19
Carrier Sense Multiple Access with Collision
Detection (CSMA/CD)

12.20
In this method, a station monitors the medium
after it sends a frame to see if the transmission
was successful. If so, the station is finished. If,
however, there is a collision, the frame is sent
again.
Example 12.5 (Minimum frame size)
A network using CSMA/CD has a bandwidth of 10
Mbps. If the maximum propagation time (including the
delays in the devices and ignoring the time needed to
send a jamming signal, as we see later) is 25.6 μs, what
is the minimum size of the frame?
Solution
The frame transmission time is Tfr = 2 × Tp = 51.2 μs.
This means, in the worst case, a station needs to
transmit for a period of 51.2 μs to detect the collision.
The minimum size of the frame is 10 Mbps × 51.2 μs =
512 bits or 64 bytes. This is actually the minimum size
of the frame for Standard Ethernet.
12.21
Figure 12.14 Flow diagram for the CSMA/CD (Page 375)
12.22
12-2 CONTROLLED ACCESS
In controlled access, the stations consult one another
to find which station has the right to send. A station
cannot send unless it has been authorized by other
stations. We discuss three popular controlled-access
methods.
Topics discussed in this section:
Reservation
Polling
Token Passing
12.23
Reservation

In the registration method, a station needs to make a
registration before sending data.

Time is divided into intervals. In each interval, a
reservation frame precedes the data frames sent in
that interval.

Figure 12.18 shows a situation with five stations and
a five-minislot reservation frame.
12.24
Figure 12.18 Reservation access method
Reservation-station needs to make a reservation before sending data
12.25
polling
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12.26
Polling works with topologies in which one device is
designated as a primary station and the other devices are
secondary stations. All data exchanges must be made through
the primary device even when the ultimate destination is a
secondary device.
The primary device controls the link; the secondary devices
follow its instructions. It is up to the primary device to
determine which device is allowed to use the channel at a
given time. The primary device, therefore, is always the
initiator of a session (see Figure 12-19).
Figure 12.19 Select and poll functions in polling access method
Polling – one device as primary station and the other device as secondary station
Select – primary device wants to send data to secondary device, secondary device
gets ready to receive
Poll – primary device solicits (ask) transmissions from secondary devices
12.27
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The poll function is used by the primary device to solicit
transmissions from the secondary devices. When the primary is
ready to receive data, it must ask (poll) each device in turn if it
has anything to send. When the first secondary is approached, it
responds either with a NAK frame if it has nothing to send or
with data (in the form of a data frame) if it does.

If the response is negative (a NAK frame), then the primary
polls the next secondary in the same manner until it finds one
with data to send. When the response is positive (a data frame),
the primary reads the frame and returns an acknowledgment
(ACK frame), verifying its receipt
12.28
Token passing – stations in network organized in a logical ring –
predecessor and successor
Token – gives station right to access the channel; needs token
management
Physical ring – station sends the token to successor
Dual ring – uses second ring which operates in reverse direction
Bus ring (token bus) - stations are connected to single cable called bus,
but make logical ring
Star ring - physical topology star, wiring inside hub makes the ring
12.29
Figure 12.20 Logical ring and physical topology in token-passing access method
12.30
12-3 CHANNELIZATION (Beyond class scope)
Channelization is a multiple-access method in which
the available bandwidth of a link is shared in time,
frequency, or through code, between different
stations.
Examples of Channelization:
Frequency-Division Multiple Access (FDMA)
Time-Division Multiple Access (TDMA)
Code-Division Multiple Access (CDMA)
12.31
Frequency-Division Multiple Access (FDMA)
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12.32
In frequency-division multiple access (FDMA), the available
bandwidth is divided into frequency bands. Each station is
allocated a band to send its data. In other words, each band is
reserved for a specific station, and it belongs to the station all
the time.
Each station also uses a bandpass filter to confine the
transmitter frequencies. To prevent station interferences, the
allocated bands are separated from one another by small guard
bands. FDMA, on the other hand, is an access method in the
data link layer. The data link layer in each station tells its
physical layer to make a bandpass signal from the data
passed to it. The signal must be created in the allocated band.
Time-Division Multiple Access (TDMA)
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12.33
In time-division multiple access (TDMA), the stations share
the bandwidth of the channel in time. Each station is allocated
a time slot during which it can send data.
Each station transmits its data in is assigned time slot.
The main problem with TDMA lies in achieving
synchronization between the different stations. Each station
needs to know the beginning of its slot and the location of its
slot.
This may be difficult because of propagation delays introduced
in the system if the stations are spread over a large area.

12.34
In TDMA, the bandwidth is just one
channel that is timeshared between
different stations.
Code-Division Multiple Access (CDMA)
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12.35
Code-division multiple access (CDMA) was conceived several
decades ago. Recent advances in electronic technology have
finally made its implementation possible.
CDMA differs from FDMA because only one channel
occupies the entire bandwidth of the link. It differs from
TDMA because all stations can send data simultaneously;
there is no timesharing. CDMA simply means communication
with different codes.
For example, in a large room with many people, two people can talk in
English if nobody else understands English. Another two people can talk in
Chinese if they are the only ones who understand Chinese, and so on. In
other words, the common channel, the space of the room in this case, can
easily allow communication between several couples, but in different
languages (codes).